MPG vs MPH

Both my wife`s 2005 XT wagon( 5 MT) and my 2009 SE (4 AT) have digital readouts in the dash indicating gas consumption. Though geared differently, both show better mileage at 70MPH than at 60MPH.This seems counter intuitive. Anybody have a good explanation?

Why does that seem counter intuitive?

 

Economy is a function of several things. From the environmental side you have contributions of rolling resistance and aero resistance. There is a critical point where rolling resistance and aero switch to take dominance. That point is fairly low, power consumption to overcome rolling resistance is a relatively constant function, while aero resistance is a cube. Once you are in the aero dominated regime, you can estimate rolling as being non-contributing.

 

Now, power consumption increases as speed increases. But engines are not perfectly efficient. They, too, increase in power efficiency across their effective operating ranges.

 

It just so happens that while it requires more power to operate at 70mph vs. 60mph, the engine is sitting at a point in it's range where it makes power more efficiently at the RPM associated with 70mph than it does at the RPM associated with 60mph.

 

This is overly simplified, but enough for you to get the gist.

Thanks

I wouldn't count on the Subie readout to be the gospel when it comes to MPG. In theory it should dividing fuel flow by wheel revs, but I've yet to see one that does this accurately. How the little magic box actually accounts for flow is uncertain to me. Yes, modern EFI systems should 'know' how much fuel is passing the injector, but this is where I believe the system falls apart. It's not like there's a fuel flow totalizer like in aircraft, it's just a presumed flow. Clogged injectors, lazy O2 senors, etc make this be more of a guess than an accurate accounting.

 

The only way to absolutely know your MPG is to divide GPS mileage by gallons at the pump.

 

That said, I don't disagree with BAC... there is a sweet spot of efficiency in every machine. Back to the aviation example, it takes way more power to fly at 50 KIAS in a 182 than at 80 KIAS. Wind resistance does increase with the square of velocity, however, so your correct to suspect parasitic drag not favoring higher speeds.

Gearing makes all the difference in the world. This is how many new V8's can achieve good MPG. Anyway I have notcied the same. I usually cruise at about 80mph and average around 24mpg.

The only way to absolutely know your MPG is to divide GPS mileage by gallons at the pump.

 

That only gives you average fuel consumption (which includes idle time at 0MPG).

 

The ideal way, would be to monitor fuel flow-rate. Not a difficult method, though not a functionality built into the car.

 

Even though GPS isn't absolute, it's the best we can really do to accurately measure distance.

 

Wind resistance does increase with the square of velocity, however, so your correct to suspect parasitic drag not favoring higher speeds.

 

Remember that you need to consider net wind speed. If you are traveling 100mph with a 100mph tail-wind, your relative wind speed is zero and the contribution to power consumption by aerodynamic resistance is zero.

 

This is almost insignificant in ground vehicles, however, since gusts parallel and opposite to vehicle travel almost never occur. But the effects of this can be seen when drafting trucks. You are in a cone of air that is traveling approximately the same speed as the truck (and, thus, the same speed as your car). So the car effectively operates in air the same speed as it's ground speed, and economy increases accordingly.

Our engines (well, any engine) carries some non-zero load just required to overcome internal resistance. It takes a descent amount of energy just to idle, circulate oil, circulate water, turn the alternator, crank windage, piston friction, valve drag, etc. This load doesn't go away when the vehicle starts moving, although it should become a lower percentage of the total output at cruise speeds. So as the wind drag increases, the ratio of productive output to waste decreases. Where this exact point is, I don't know.

 

Outside of this conversation, the other thing to keep in mind with 'modern' engines is that fuel consumption goes to zero (not idle) at closed throttle above (roughly) 1,100 rpm. So don't ever feather the throttle coming to a stop. Keep it in gear (MT) and let the trans drive the motor, the fuel flow will completely stop and will resume as soon as the revs drop below 1,100 to keep the engine from stalling. For this reason, I can achieve higher MPG in the hills than on the flats. Counter-intuitive, but try it. You just have to come completely off the gas for this to work.

The readouts we have on our dash are calculated using IDC. Bigger the injector you have, the less duty cycle necessary so the ECM is fooled into thinking it's using less fuel than it actually is so it registers a higher MPG number for you on your display. Hence why, with my 850cc injectors, my car will read 30-37 on highway when in actuality it's getting upper 20's.

 

Just something to add into your thoughts

its just funny that all of the "economy" news reports say 60-65 give the best mileage. MALARKEY!

Well damn, I better speed!

Why not graph the readouts from 55 to 80 in both cars?

 

The problem is going to be that one of readings don't really prove much. You need a better sample size. In theory going faster reduces MPG. In all my experience 55mph in the 5th or 6th gear gets the best mpg.

I seem to only get decent MPG 35-55 MPH. Under 35 or above 60 usually results in low MPG. It's only decent when the roads are flat which isn't often here

From my spreadsheet tracking mileage over 2 years, since day 1

 

I seem to only get decent MPG 35-55 MPH. Under 35 or above 60 usually results in low MPG. It's only decent when the roads are flat which isn't often here

 

Exactly the same here. 45-50 mph is the sweet spot in my car. Anything over 70mph and mileage goes to shit. I have 740 injectors and I can get an idea within a MPG or two just taking 33% off of what my trip computer reads.

From my spreadsheet tracking mileage over 2 years, since day 1

 

[ATTACH]135581[/ATTACH]

 

wheres the rest of it? i know youve tracked faster than 65

Those are average speeds across the entire tank. So I do the miles in a tank divided by how long (time) the tank lasted.

 

Does anybody else here have a ScanGuage? Because someone could do an MPG vs MPH test for our cars like they do on EcoModder

I agree with BAC, I think it has to do with what ever rpm your motor is in and the amount of boost/lag you are on correlated with your rpms.